starship: add enableInteractive option for fish

Some fish plugins such as https://github.com/acomagu/fish-async-prompt
require that starship be initialized as non-interactive.

When the `programs.starship.enableInteractive` option is enabled,
starship is initialized at the end of the init script, outside the
interactive block.

tests/modules/programs/starship/default.nix

@@ -2,4 +2,6 @@
   starship-settings = ./settings.nix;
   starship-fish-with-transience = ./fish_with_transience.nix;
   starship-fish-without-transience = ./fish_without_transience.nix;
+  starship-fish-with-interactive = ./fish_with_interactive.nix;
+  starship-fish-without-interactive = ./fish_without_interactive.nix;
 }

tests/modules/programs/starship/fish_with_interactive.nix

@@ -0,0 +1,27 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs = {
+      fish.enable = true;
+      starship.enable = true;
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/fish/config.fish
+
+      export GOT="$(tail -n 5 `_abs home-files/.config/fish/config.fish`)"
+      export NOT_EXPECTED="
+      if test \"\$TERM\" != dumb
+          /home/hm-user/.nix-profile/bin/starship init fish | source
+
+      end"
+
+      if [[ "$GOT" == "$NOT_EXPECTED" ]]; then
+        fail "Expected starship init to be inside the 'is-interactive' block but it wasn't."
+      fi
+    '';
+  };
+}

tests/modules/programs/starship/fish_without_interactive.nix

@@ -0,0 +1,43 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  config = {
+    programs = {
+      fish.enable = true;
+
+      starship = {
+        enable = true;
+        enableInteractive = false;
+      };
+    };
+
+    nmt.script = ''
+      assertFileExists home-files/.config/fish/config.fish
+
+      export GOT="$(tail -n 5 `_abs home-files/.config/fish/config.fish`)"
+      export EXPECTED="
+      if test \"\$TERM\" != dumb
+          eval (/home/hm-user/.nix-profile/bin/starship init fish)
+
+      end"
+
+      export MESSAGE="
+      ==========
+       Expected
+      ==========
+      $EXPECTED
+      ==========
+         Got
+      ==========
+      $GOT
+      ==========
+      "
+
+      if [[ "$GOT" != "$EXPECTED" ]]; then
+        fail "$MESSAGE"
+      fi
+    '';
+  };
+}